Genetic Disorders

The
Table
below
lists several genetic disorders caused by mutations in just one gene. Some of the disorders are caused by mutations in autosomal genes, others by mutations in X-linked genes. Which disorder would you expect to be more common in males than females? You can watch a video about genetic disorders caused by mutations at this link:
http://www.pbs.org/wgbh/nova/programs/ht/rv/2809_03.html
.

soft bones that easily become deformed, leading to bowed legs and other skeletal deformities

X-linked dominant

Hemophilia A

reduced activity of a protein needed for blood clotting

internal and external bleeding that occurs easily and is difficult to control

X-linked recessive

Few genetic disorders are controlled by dominant alleles. A mutant dominant allele is expressed in every individual who inherits even one copy of it. If it causes a serious disorder, affected people may die young and fail to reproduce. Therefore, the mutant dominant allele is likely to die out of the population. A mutant recessive allele, such as the allele that causes sickle cell anemia (see
Figure
below
and the link that follows), is not expressed in people who inherit just one copy of it. These people are called
carriers
. They do not have the disorder themselves, but they carry the mutant allele and can pass it to their offspring. Thus, the allele is likely to pass on to the next generation rather than die out.
http://www.dnalc.org/resources/3d/17-sickle-cell.html

Sickle-Shaped and Normal Red Blood Cells. Sickle cell anemia is an autosomal recessive disorder. The mutation that causes the disorder affects just one amino acid in a single protein, but it has serious consequences for the affected person. This photo shows the sickle shape of red blood cells in people with sickle cell anemia.

Mistakes may occur during meiosis that result in
nondisjunction
. This is the failure of replicated chromosomes to separate during meiosis (the animation at the link below shows how this happens). Some of the resulting gametes will be missing a chromosome, while others will have an extra copy of the chromosome. If such gametes are fertilized and form zygotes, they usually do not survive. If they do survive, the individuals are likely to have serious genetic disorders.
Table
below
lists several genetic disorders that are caused by abnormal numbers of chromosomes. Most chromosomal disorders involve the X chromosome. Look back at the X and Y chromosomes and you will see why. The X and Y chromosomes are very different in size, so nondisjunction of the sex chromosomes occurs relatively often.
http://learn.genetics.utah.edu/content/begin/traits/predictdisorder/index.html

Diagnosing Genetic Disorders

A genetic disorder that is caused by a mutation can be inherited. Therefore, people with a genetic disorder in their family may be concerned about having children with the disorder. Professionals known as
genetic counselors
can help them understand the risks of their children being affected. If they decide to have children, they may be advised to have
prenatal
(“before birth”) testing to see if the fetus has any genetic abnormalities. One method of prenatal testing is
amniocentesis
. In this procedure, a few fetal cells are extracted from the fluid surrounding the fetus, and the fetal chromosomes are examined.

Treating Genetic Disorders

The symptoms of genetic disorders can sometimes be treated, but cures for genetic disorders are still in the early stages of development. One potential cure that has already been used with some success is
gene therapy
. This involves inserting normal genes into cells with mutant genes. At the following link, you can watch the video "Sickle Cell Anemia: Hope from Gene Therapy," to learn how scientists are trying to cure sickle-cell anemia with gene therapy.
http://www.pubinfo.vcu.edu/secretsofthesequence/playlist_frame.asp